1. Field
The present application describes test and burn-in equipment for integrated circuit diagnostics and manufacture and in particular, a system for handling multiple integrated circuits with tight control of operational parameters and instruction execution.
2. Background
Many IC (Integrated circuit) chips, including CPU's (Central Processing Units) and other processors are subjected to diagnostic tests and burn-in before they are put into regular use. The diagnostic and burn-in processes vary with different chips and different manufacturers. Burn-in may involve driving the chip at low clock speeds and elevated temperatures and voltages through a series of test sequences. For complex chips, burn-in can take many hours before the chips are sufficiently tested and conditioned for commercial use. Special equipment is required to maintain the appropriate clock rates, temperatures, voltages and sequences. This equipment may be different for different IC's.
Burn-in equipment technology changes rapidly to keep up with increases in power, speed and density for the IC's upon which they operate. Some new CPU's include multiple cores and caches and need separate voltage supplies for different sections of the chip. Accordingly, burn-in steadily increases in cost and complexity.
In addition, burn-in equipment is replaced or upgraded to meet the demands of new chip designs. In order to reuse existing equipment, some existing burn-in tools can be upgraded by depopulation, i.e. using the same equipment to test a smaller number of CPU's. This can allow multiple clock rate and power supply demands to be serviced without replacing the equipment. However, factory throughput is greatly reduced, or the amount of floor space required to burn-in the same number of CPU's is increased.
Accurate and frequent measurement of the operating physical parameters of the IC's allows for more precise control of these parameters. For example, during burn-in testing, a very small increase in voltage or temperature can significantly reduce the time required to complete a burn-in cycle and therefore increase factory throughput. On the other hand, too high a voltage or temperature can destroy the IC. In addition, with semiconductor chips, as the temperature increases, resistance drops, increasing the current for a fixed voltage. The increased current increases the temperature inducing a nonlinear feedback effect. Operation near the temperature and voltage limits, therefore requires an increased precision in measurements of the chips' physical parameters and a faster response time. As the geometry of chip architecture grows smaller and smaller, the chips become more susceptible to gate oxide breakdown, requiring further increases in precision and speed.
In order to regulate the voltage on a chip during burn-in, typically, the voltage at the output of the voltage regulator module that powers the chips is sensed. This voltage may be different than at the chip and it may change more slowly than the voltage at the chip. In addition, due to variations in signal path, chip temperature, and chip constitution, the voltages differences may differ at different chips. As a result, chip voltage cannot be accurately determined at a voltage regulator module output.
As burn-in currents are increased, so are voltage drops along the power path and the corresponding amount of heat that must be dissipated. The increased voltage drops increase the difference between the voltage regulator module voltage and that of each chip. In order to prevent the chips from being overstressed during burn-in, the set point voltage is lowered at the voltage regulator module. However, this increases the burn-in time. As a result more burn-in chambers are required to achieve the same production volume.
During diagnostic and burn-in processes a single test signal generator can be used to send instruction sequences to a group of IC's at the same time, or in quick succession. Many IC's require these signals in a gunning transceiver logic (GTL) level and the signals are often transmitted as low voltage transistor-to-transistor logic (LVTTL). While LVTTL signaling may work well over the short, low power, protected distances typical in operational components, in a system designed to test a group of IC's these signals must run longer distances across spaces with higher amounts of interference. As speed, power and heat increase, LVTTL signals are more frequently distorted or changed, increasing communication error rates. The distorted signal could have a significant effect on the IC's internal input/output structure and could lead to permanent damage if not controlled properly. In addition, such errors may result in non-genuin failure, which requires that sequences be repeated, increasing the amount of time required for execution.